The invention relates to aluminum production plants using igneous electrolysis according to the Hall-Héroult process. It especially relates to handling equipment used in said plants to handle the anodes while they are being replaced in the electrolysis cells.
Metallic aluminum is produced industrially by igneous electrolysis, namely by electrolysis of alumina in solution in a molten cryolite bath, known as an electrolyte bath, using the well-known Hall-Héroult process. The electrolyte bath is contained in tanks, called “electrolysis pots”, comprising a steel container, which is coated on the inside with refractory and/or insulating materials, and a cathode unit located at the bottom of the pot. Anodes, typically made of is carbonaceous material, are partially immersed in the electrolyte bath. The assembly formed by an electrolysis pot, its anodes and the electrolyte bath is called an electrolysis cell.
The plants contain a great number of electrolysis cells laid out in line, in buildings called potrooms, and electrically connected in series using connecting conductors, in order to make the best use of the floor area of the plants. The cells are generally laid out so as to form two or more parallel lines which are electrically linked to each other by end conductors. The electrolysis current thus cascades from one cell to the next. The cells include series of anodes, each of which is in the form of an anode block connected to a metal rod, called an anode rod. The rod is generally rectangular in section so that one of its faces can be kept firmly in contact with an anode frame which extends over the entire length of the cell and carries the electrolysis current.
When operating, an electrolysis plant requires work on the electrolysis cells, in particular replacement of spent anodes by new ones, sampling of molten metal in the cells and sampling or top-ups of electrolyte. The most modern plants are equipped with lifting and handling units comprising an overhead travelling crane, which can be moved above and along the electrolysis cells, and a carriage on which is fixed a tending module comprising a frame that can be fixed to said carriage and a handling and servicing unit including several tools, such as shovels, clamps and hoists.
The tool designed for handling and moving loads such as anodes comprises a clamp suitable for grasping the load, called “a handling clamp” or an “anode clamp” During anode handling operations, this clamp is placed near the rod of the anode to be grasped, then closed on the rod so that the anode can be raised, moved, then positioned or put down at the desired place.
When replacing an anode on an aluminum electrolysis pot, it is often observed that the electrical contact between the anode rod and the anode frame is not satisfactory. But it is important for electrical contact at this level to be excellent, i.e. it must give a voltage drop, known as an “anode drop”, that is as low as possible: an anode drop of 15 mV results in a significant loss of productivity for the pot, because of both the reduction of that part of the electrical power which is actually devoted to electrolysis, and the longer time taken for the anode to come up to operating power. Lastly, a very poor contact can lead to the creation of electric arcs likely to damage the contact surfaces.
Moreover, an insufficient electrical contact between the anode and the anode frame can be observed only several hours after replacing the anode, because it takes several hours of heating the anode unit before the electric current reaches its full capacity. So, as things stand today, over ten hours after fitting the anode, typically 16 hours, an operator comes to read the difference in voltage between the anode framework and the anode rod: if this voltage difference is higher than a critical value, the electrical contact between the rod and the anode frame needs to be modified.
With a view to improving safety, research has been carried out to limit the causes of staff having to work near danger zones. Among these causes is checking anode drop. It has therefore been attempted to eliminate such work, without giving up checking that the current passes properly between the anode frame and the anode rod at all times.